Phosphotungstic Acid Modified Expanded PTFE Based Nafion Composites

Journal of New Materials for Electrochemical Systems 14, 217-222 (2011) © J. New Mat. Electrochem. Systems

Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites

K. Pattabiraman and K. Ramya

Centre For Fuel Cell Technology, ARC- International, IIT M Research Park, Phase I, 2nd Floor, No.6 Kanagam Road, Taramani, Chennai 600113, India.

Received: May 06, 2011, Accepted: May 30, 2011, Available online: July 06, 2011

Abstract: Composite membranes have been prepared by impregnation of PFSI( perfluorosulphonic acid ionomer) solution (Nafion®) into the expanded polytetrafluoroethylene (EPTFE) matrix. The matrix was modified by Na/Naphthalene treatment and with phosphotungstic acid (PTA). The matrix modifications helped in reducing the contact angle thereby improving the impregnation process by reducing the hydrophobicity of the matrix. The fuel cell made with these composite membranes showed that the performance of PTA modified matrix is comparable to that of Na/ naphthalene modified matrix. Further the performances of matrix modified membranes were higher than that of the as received matrix-composites. PTA treatment offers a simple method of matrix modification to achieve high performance.

Keywords: Polymer electrolyte membrane fuel cells, Composite ionomers, expanded PTFE, Phosphotungstic acid, surface modification

1. INTRODUCTION high temperature performance of the most commonly used perfluorosulphonic acid based polymer electrolyte in fuel cells Perfluorosulphonic acid ionomers (PFSI) are fast ion conductors [13,14]. Malhotra and Dutta [15] showed that incorporation of and are widely used as an electrolyte in many electrochemical inorganic solid acids (phosphotungstic acid) in Nafion resulted in devices [1]. Recent literature reports have shown that composite improved water retention capacity due to additional acid sites and membranes of PFSI offer many advantages in terms of lower cost, good fuel cell performance at elevated temperatures. Kannan et al higher thermal stability and good mechanical strength [2], im- [16] have prepared modified Nafion membranes with phospho- proved durability and performance [3], reduced swelling character- silicate for improved water retention and proton conduction. They istics [4] etc. A composite membrane prepared using a porous also found that samples with higher phosphate content exhibited framework substrate with the pores filled with a PFSI such as high water uptake and proton conductivity. Nafion offers all the benefits described above [5,6]. Nafion has a Literature studies thus cite the advantages of using hydrophobic tetrafluoroethylene backbone and hydrophilic side Nafion/EPTFE composite membranes and the use of heteropolya- chains that aggregate making it difficult to fill the porous EPTFE cids as additives for improvement in properties. Further surface substrates with Nafion solutions. Various methods to improve modified EPTFE have been shown to have better wetting charac- impregnation include use of solvents having solubility parameters teristics and improved properties due to better interactions between suitable to backbone/ ionomer[2,7], use of surfactants [8], surface the substrate and Nafion. As EPTFE substrate is very stable it modification of EPTFE through etching [9,10] etc. requires a strong radical ion (sodium Naphthalene radical) to ab- Heteropolyacids (HPAs) are the most attractive inorganic modi- stract the fluoride ion from backbone. The process requires the use fiers because these materials in crystalline form are highly conduc- of a number of solvents and the localized heat generated during tive [11] and thermally stable. The use of heteropolyacids in defluorination may damage the membrane. A simpler process for preparation of composite polymer electrolytes for fuel cell offer surface modification would hence be advantages. This paper sug- many advantages as they improve conductivity and decrease the gests a non-reactive surface modification method using phospho- dependence of the membrane on humidity for optimum perform- tungstic acid. The EPTFE substrate treated with PTA solution ance [12]. In addition, they have also been proven to improve the would have a coating of PTA molecules in the pores of the sample. *To whom correspondence should be addressed: Email: [email protected] Due to good interactions between the PTA and Nafion all the Phone: +91-44-66632710 , Fax: +91-44-66632702 pores of substrate would get coated and an improved membrane

217 218 K. Pattabiraman and K. Ramya / J. New Mat. Electrochem. Systems would be obtained. The results of these modifications are presented in this paper. To the best of our knowledge this is the only paper where PTA has been used first to treat the matrix to be coated with Nafion later.

2. EXPERIMENTAL 2.1. Preparation of Nafion/EPTFE composites Porous EPTFE ( Yeu Ming Tai Chemical Industrial Co., Ltd., Taiwan, thickness 25±5µm, porosity 85%) was first washed with alcohol and dried in oven. The EPTFE film was then mounted on a frame and soaked in acetone for 30 min. Nafion solution (5%) obtained from Dupont Co, with an equivalent weight of 1000 was used for impregnating the porous EPTFE film. To minimize the aggregation of Nafion in the solution it was mixed with a high boil- ing solvent ( dimethylformamide / etylene glycol) to make the solution into 2.5% of polymer in the solvent. The Nafion loading in the porous EPTFE film was maintained constant at 2mg/cm2 for all the membranes. Impregnation was carried out in steps to ensure that all the pores of the membrane were fully blocked to avoid gas cross- over. Membrane post treatment procedure included pressing the membranes at 30kg/cm2 at 120°C for 3 minutes followed by annealing. The membranes were soaked in water for 24 hrs and then in dilute

H2SO4 for 4 hrs. The membranes were washed free of acid and used for further studies. The average thickness of the membranes was 27±5µm. For preparation of etched membranes the procedure adopted by Tang et al [9] and Jie et al [10] were used. The etched membranes were impregnated with Nafion using the procedure mentioned above. Fig 1 shows the schematic for the preparation of PTA modified membranes. The impregnation procedure and the post treatment procedures followed were the same as that for other mem- Figure 1. Schematic for the preparation of PTA modified PTFE branes. composite

2.2. Characterization of Nafion/EPTFE composites The morphology of the surface of the composite membranes was studied using scanning electron microscope (SEM). The surface of fabrication of the MEA. the sample was coated with gold before the morphology of the The composite membranes prepared by the above techniques membranes was observed. EDAX analysis was carried out to study were used for the preparation of membrane electrode assemblies. the various elements present on the surface. FTIR studies were The membrane electrode assemblies were made by a proprietary carried out to study the functional groups present and the interac- technique developed in our lab using 40% Pt/C( Arora Mathey, tion between the various groups present. Thermogravimetry studies India), 5% Nafion Solution (DuPont, USA) and Carbon substrate were carried out to study the thermal stability of the various mem- from Ballard, USA. The catalyst layers were coated on a Gas diffu- branes prepared. Contact angle measurements were carried out to sion layer by brush and the catalyst loading (total) was 1mg/sq cm. 2 study the effect of various surface modifications on the porous The performance data were obtained from a 6.75 cm active area PTFE substrate. Conductivity measurements were carried out in the cell. Pure hydrogen and air were used as fuel and the oxidant. The transverse direction using two probe techniques. The membrane cells were operated at ambient pressure conditions. The inlet gases resistance was obtained by AC impedance technique in the fre- were humidified to temperatures higher by 10°C and 5°C than cell quency range of 100kHz to 100 Hz. temperature for hydrogen and air respectively.

2.3. Gas permeability and Fuel cell polarization 3. RESULTS AND DISCUSSION studies Three types of composite membranes based on Nafion/EPTFE Gas permeability across the membranes were tested by assem- were prepared. This included impregnation of pure EPTFE with bling the membrane in a fuel cell set up and passing the gas on one Nafion solution, fabrication of etched EPTFE/ Nafion composite 2 side of the cell. The test consisted of applying a 1kg/cm differen- and third non-interacting PTA modified EPTFE/Nafion composite. tial over pressure on one side while capping the other inlet and Although etching is a good surface modification method, it requires leading the outlet to a water reservoir[3]. If any bubble was ob- the use of Na/naphthalene radical synthesis and many solvents. The served in the water reservoir the membranes were not used for process may also not lead to uniform etching when large area mem- Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites / J. New Mat. Electrochem. Systems 219

a) b)

c) d)

Figure 2. SEM pictures of a) Plain EPTFE, b) Etched EPTFE and c) PTA modified EPTFE d) Nafion coated EPTFE e) Transverse section of Nafion- ePTFE

branes have to be processed. A non-reactive surface modification son to that of pure PTFE film the pores have become smaller. The method is suggested and the properties are compared with that decrease in the pore size has been reported to be due to structure obtained in other methods. The PTA modification was expected to and stress change as a result of Na/Naphthalene treatment [9]. The improve coatability and reduce interfacial resistance due to de- reduction in pore size was also reported to be advantageous for crease in contact angle as a result of PTA coating on the pores and impregnation of the pores due to higher capillary force. The smooth surface of the matrix when treated with a dilute solution of PTA in surface of the etched film shows that the film has not been dam- a wetting liquid and due to interaction of Nafion with PTA. aged either in the surface or in cross section due to Na/naphthalene treatment. Fig 2c shows the surface of the PTA modified EPTFE 3.1. SEM-EDAX analysis film. The pore size in this case has decreased due to the presence of Fig 2 shows the SEM picture of different types of EPTFE mem- PTA. As the solution of PTA has low viscosity it coats the pores brane. The surface of the pure EPTFE ( Fig 2a) film shows the with it and decreases the pore size. The PTA molecules are held presence of fibers and knots and these enclose the micropores. The within the matrix as they are entangled in the fibers and knots of surface of the etched EPTFE( Fig 2b) film shows that in compari- the EPTFE film. The EDAX analysis confirmed the presence of 220 K. Pattabiraman and K. Ramya / J. New Mat. Electrochem. Systems

Figure 3. IR Curves for Plain, Etched and PTA modified mem- Figure 4. TGA curves for various membranes branes

3.3. Thermogravimetric analysis for the mem- PTA on the surface as both tungsten and phosphorous could be branes detected on the surface while the EDAX of pure PTFE showed Fig 4 gives the TGA curves for the various EPTFE membranes only the presence of carbon and fluorine. The surface of the Nafion prepared. Pure EPTFE film is very stable and starts decomposing coated films is smooth, uniform and without any cracks( fig 2d). above 530°C only. As these films are hydrophobic moisture loss SEM picture of the transverse section shows that all the pores are below 100°C are also not seen. EPTFE-Nafion composite shows filled ( fig 2e). The EDAX of Nafion coated films shows the pres- transitions due to sulphonic acid groups above 300°C followed by ence of O and S along with carbon and fluorine. The surface of the the main chain scission above 450°C. The etched EPTFE films PTA modified film shows the presence of W and P even after post show transition at the temperatures higher than that of Pure com- processing indicating that there is stabilizing interactions between posite membrane with peak temperatures at 348°C for sulphonic Nafion and PTA. Also, PTA has not been leached out during the acid groups and 569°C for the main chain. The increase in degrada- post treatment of the membranes (processing with acid and that tion temperature could be due interaction between the sulphonic with water). acid groups of the PFSI and the hydrophilic groups introduced due to Na/ naphthalene etching. The slight changes in the transition 3.2. FTIR analysis of EPTFE membranes temperature between the etched and the pure EPTFE based iono- Fig 3 shows the FTIR curves for Plain EPTFE, etched EPTFE mers suggests that Na/Naphthalene treatment have been carried out and PTA modified EPTFE. The plain EPTFE shows characteristic only to the moderate extent and no large scale degradation in the peaks in the region 503 cm-1 due to -CF2 rocking mode, 620-640 main chain have occurred. These results also concur with the SEM cm-1 due to -CF2 wagging, 1150 cm-1 due to -CF2 stretching and and IR data. The PTA modified EPTFE shows transitions at peak 1240 cm-1 due to –CF3 stretching. The surface of the etched PTFE temperature of above 390°C. This could be due to stabilizing bond shows peaks at 3500 cm-1 due to – OH groups, at 2950 cm-1 and formed between sulphonic acid groups of the PFSI and the acid 1739 cm-1 due to – CH3 and -CO groups. The presence of these functionality of PTA. groups indicate less hydrophobicity of the material and easy coatability with solvents like dimethyl formamide, dimethy- 3.4. Contact angle measurement lacetamide , N methylpyrolidone etc. The PTA modified EPTFE Table 1 gives the contact angles measured for various mem- shows bands due to O-H bending at 1630 cm-1, P-O stretching at branes. The plain EPTFE is very hydrophobic and has a contact 1080cm-1, W=O stretching at 987 cm-1, W-O-W stretching at 886 angle of 153°. Treatment with Na/naphthalene renders the surface cm-1 and 810 cm-1. As PTA is a non- interacting material with of EPTFE less hydrophobic as seen by the presence of –OH, -CH3 EPTFE, no shifts in the EPTE bands are seen. and –CO groups in the FTIR study. As a result the contact angle

Table 1. Properties of the EPTFE membranes Thickness of Matrix Contact angle(/°) of Conductivity (mS/cm) of S. No Membrane type ( µM) the matrix ionomer coated membranes 1 Plain EPTFE 25±5 153 16 2 Etched EPTFE 24±5 84 30 3 PTA modified EPTFE 27±5 132 23 Phosphotungstic Acid Modified Expanded PTFE based Nafion Composites / J. New Mat. Electrochem. Systems 221

Figure 5. H2/Air Polarization curves for Pure EPTFE

Figure 7. H2/Air Polarization curves for PTA modified composite membrane

Figure 6. H2/Air Polarization curves for Etched EPTFE

decreases and was found to be 84° for the etched sample. This improves the coatability with Perfuorosulphonic acid polymer that has Figure 8. Comparative Polarization curves for various membranes hydrophobic backbone and hydrophilic pendent groups. PTA modified EPTFE has a coating of PTA on the surface of EPTFE. As a result the contact angle should decrease. The results concur with the hypothesis and the contact angle was found to be 132°. or improved coating techniques using surface modification meth- The decrease in the contact angle along with the ability to interact ods. Thus the conductivity value for etched EPTFE membranes that with Nafion( as indicated by TGA studies) is expected to provide have the lowest contact angle (indicating better interaction between membranes with low interfacial resistance and performance. the substrate and PFSI) are the highest. PFSI coated on the plain EPTFE membranes have the lowest conductivity. The contact angle 3.5. Conductivity analysis for PTA modified membrane is in between that of etched and the Table 1 gives the through plane conductivity values for various plain EPTFE membrane. The PTA modified membrane has higher Nafion coated membranes at 30°C. The composite membranes conductivity than that of as received EPTFE coated membrane due based on EPTFE and PFSI have an interface between the two (the to its better pore filling ability. matrix being non conducting). The conductivity values were found to be a function of ability to completely fill Nafion in the pores of 3.6. Fuel cell Performance of the membranes the EPTFE matrix (to reduce the interfacial resistance). This de- The current-voltage (I-V) curves for single cells fabricated with pends on the either the interaction between the matrix and the PFSI various PEM’s are shown in Fig 5-8. The open circuit voltage for 222 K. Pattabiraman and K. Ramya / J. New Mat. Electrochem. 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5. ACKNOWLEDGEMENT The authors would like to thank Dr.G.Sundararajan, Director, ARCI, Hyderabad and Dr. K S Dhathathreyan, Associate Director, CFCT for supporting this work and Department of Science and Technology, Government of India for financial support. The authors would like to thank Dr.Y.S.Rao, Dr. R.Subasri and Dr. G. Ravichandra of ARCI for their help in characterizing the membranes.